This invention relates to pneumatic valves, and more particularly to a combined peak flow and decay rated valve assembly suitable for use in a medical respirator.
It is of great importance in respirator therapy to carefully control the gas flows to avoid injuring the patient and to ensure that a maximum therapeutic effect is achieved. To this end various flow control valves are normally installed in the conduit system which connects the respirator air supply with the patient. One of these valves is typically a peak flow control which restricts the maximum volumetric flow of gas to the patient to a predetermined amount, in order to avoid expanding the patient's lungs beyond a safe and prudent level. The peak flow rate is normally reached during an early stage of inspiration, followed by a gradually decreasing intake of air until the patient is ready to exhale. Another valve is frequently employed to control the rate at which the inspiratory air flow tapers off, with the optimum decay rate depending upon the type of therapy administered. The decay rate valve normally employs a separate restriction in the air flow conduit, which restriction varies in inverse proportion to the pressure in the conduit leading to the patient. Thus, as the pressure in the patient conduit increases, indicating that the patient's air needs are being satisfied, the decay rate restriction drops and partially nullifies the decrease in flow rate that would otherwise accompany the increase in patient pressure. By an appropriate selection of the initial decay rate restriction, the decay rate can be controlled from a rapid decay to an almost square wave characteristic in which the air flow remains near the maximum flow level until the end of the inspiration is approached. For example, if a rapid decay is desired, the decay rate valve would be set to a fairly high initial restrictive value. If a slow decay is desired, the valve is set to a fairly low initial restrictive value which permits a greater increase in conduit pressure before significantly closing the conduit.
While the above decay rate valve does enhance the overall control which can be be achieved, it also introduces an additional restriction in the flow line which can effect the maximum flow level in undesired ways. In the case of a high initial restriction, in addition to having the desired effect on the decay rate the valve would also increase the total restriction in the patient supply conduit so as to reduce the peak flow value. To avoid introducing an error into the peak flow valve, some compensating adjustment of the peak flow valve would then have to be made. Each time the decay rate valve is adjusted thereafter, the peak flow would be further altered.
Various valves are known which involve more than one flow restricter, and in which an alteration of one restricter changes the gas flow through the other. U.S. Pat. No. 2,608,209 to Bryant and U.S. Pat. No. 2,951,501 to Thylefors disclose valves which are designed to provide a constant volumetric flow despite variations in input pressure. In each of these patents a manually adjustable downstream valve is used to establish a desired flow rate, while a spring or diaphragm-biased upstream valve filters out incoming pressure variations. The two valves act independently of each other, and only the downstream valve is adjustable.
U.S. Pat. No. 2,917,069 to Lundy et al discloses another type of valve which is both rotatably and axially adjustable by separate controls. Coarse adjustments are made by rotating the valve, followed by a fine adjustment made by moving the valve axially. A somewhat similar concept is utilized in patent application Ser. No. 677,344 filed Apr. 15, 1976 and assigned to the assignee of the present invention. In this invention a cylindrical valve is manually rotated to adjust the gas flow level, while the rotatable part is affixed to a diaphragm for axial movement which restricts the valve opening when the flow rate decreases. The object of the latter invention is to maintain a substantially constant pressure drop across the valves despite variations in flow rate.
While each of the above patents and patent application disclose multiple valve devices, none of them provides the combination of (a) a peak flow control and (b) a flow decay rate control which can be adjusted without altering the peak flow rate.